Updated June 23, 2015.

Determining your due date can seem like a maze these days. How do you do it and what does it all mean?

Let's take a look at the history of determining due dates and how it affects when your baby will be born.

Dr. Naegele, circa 1850, determined that the average length of human gestation was approximately 266 days from conception. He assumed that the average woman had cycles that lasted 28 days and that she ovulated on Day 14 of her cycle.

He used his data to come up with a mathematical calculation for due dates:

  ((LMP + 7 days) - 3 months) = Due Date
EX: ((January 1, 1996 + 7 days) - 3 months) = October 8, 1996

However, Dr. Naegele did not consider certain factors in his calculation. For example: Not every woman ovulates on Day 14. Other situations that he did not factor in were ethnicity, parity (how many successful pregnancies), prenatal care, better nutrition, and screening factors.

Today we still use Naegele's rule to determine due dates. However, there is a new rise to the discussion of the accuracy of Naegele's findings. With the advent of true prenatal care, midwives and physicians are helping women educate themselves about risk factors, nutrition and prenatal screening. This extends the lengths of gestation for many women.

One study indicates that we need to add 15 days to the Naegele EDC for Caucasian, first time moms, and 10 days for Caucasian moms having subsequent children. African American and Asian women tend to have shorter gestations.

Nowadays, we use ultrasound, when available or if there is a question of menstrual history. Ultrasound can be an effective way of dating a pregnancy, but this accuracy is lost if not performed in the first half of pregnancy. And ideally, the earlier these are performed the better. The best are done in the first trimester.

Most authorities agree that there are many ways to date a pregnancy, and that not just one factor should be used to determine the final due date. Other events to factor in are:

• Quickening (first time mom feels the baby move)
• Fetal heart tones (FHT) heard through doppler and stethoscope
• Fundal height (Measurement of the uterus done throughout pregnancy)

Keep in mind that due dates are estimates of when your baby will arrive. We generally consider the normal time frame to be two weeks before your due date, until two weeks after your due date.

Changing Due Dates

I should also take a minute to talk about changing due dates. Many times, someone will give you a due date based on one method of calculation and then turn around and try to change it based on another calculation. The most accurate due dates are going to be those based on very early ultrasound and on ovulation or last menstrual period dates that are accurate.

Some practices have been known to try to alter a due date based on an ultrasound done at 20 weeks, but these are less accurate. They are less accurate because at that point, shifting the due date based on the size will mean incorporating small variances in size that have more to do with how tall the parents are, for example, which is not the strength of the guesstimation. Earlier in pregnancy, there is less room for these individual variances, which is why they are better at predicting when your baby will be born.

And for fun, some moms want to use their due date to count backwards and see when they conceived.

Just remember, no matter how many times your neighbors, strangers, mothers, etc. ask you when you are due, smile, because only the baby really knows.